I'm wondering where I can download the source code of the kernel of Mac OS X Mountain Lion 10.8.2: Darwin 12.2.0.
I googled it, but its hard to find. And yes, I tried http://www.opensource.apple.com.
You are looking in the correct spot. I haven't seen the wiki updated since Lion - but in the past, the comprehensive build scripts were packaged as part of the darwinbuild project.
The source for 10.8.2 is listed at http://www.opensource.apple.com/release/mac-os-x-1082/ and you might just try grabbing the latest darwinbuild from MacPorts rather thab building it from source to see if it can help you pull a newer branch corresponding to 10.8 even though the documentation is not written to indicate if the Mountain Lion specific branch steps are complete (or even require anything different than just taking the current trunk code).
I found it. Actually, the Darwin kernel is named
It can be downloaded here: http://www.opensource.apple.com/tarballs/xnu/xnu-2050.18.24.tar.gz.
Source code can be browsed online here: http://www.opensource.apple.com/source/xnu/xnu-2050.18.24/